Recent data suggest that aminergic neurotransmitters may not be a factor in plasticity and establishment of synaptic connections. However, since each neuron may well possess in addition to an aminergic transmitter a more specific peptidergic modulator the question arises whether peptides and their transsynaptic receptors may be important in the establishment of specific neuronal connections during development. The arcuate nucleus is chosen to study this question because of the existence of at least one defined peptide originating in its somata and because of the relative ease with which its secretory stages can be modified under physiological conditions. We will correlate by light and electron microscopic immunocytochemistry the sequence of appearance of luteinizing hormone-releasing hormone neurons and their receptors with the establishment of specific neuronal connections. Attempts will be made to influence the development of these connections by: (a) modification of their efferent receptors; (b) modification of their afferents and (c) modification through effects of LHRH. The information gathered may show whether development of neuropeptides and their receptors are linked phenomena, whether receptors determine the direction of nerve sprouting, and whether peptidergic stimuli participate in modulating sexual dimorphism during development. The data as a whole may contribute to the notion that peptides are modulators of neurotransmission which may facilitate or inhibit an aminergic neurotransmitter stimulus and which may confer permanence or memory to a response.